It is a windows down, crank it up kind of song that just seems to suit the season (at least to me).
Under the Boardwalk reminds me of the beach - which is the BEST place to be in the summer. It also brings me back to the days of summer romances, which are pretty nice memories.
Any song that has "vacation" in the title is sure to be a summer anthem! Although I really don't get to do a summer vacation much anymore, it is also one of those songs that you can turn up really loud and sing along to and takes you back to summer vacation when you were a kid - SUPER FUN!